The present invention relates to the field of wavelength multiplexed optical communications systems and, in particular, to systems for the switching of traffic between different wavelength multiplexed channels.
Optical communications systems are a substantial and fast-growing constituent of communications networks. The expression xe2x80x9coptical communications systemxe2x80x9d, as used herein, relates to any system that uses optical signals to convey information across an optical medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, local, metropolitan and wide area networks (LANs, MANs and WANs). Optical systems are described in Gowar, Ed. Optical Communications Systems, (Prentice Hall, New York). Currently, the majority of optical communication systems are configured to carry a single optical channel in a relatively broad wavelength spectral band over an optical path consisting of one, or a series of, optical guides. To convey increased quantities of information, e.g. from a plurality of sources, wavelength division multiplexing (WDM) is now used. In a WDM system a plurality of optical signals, each typically having a narrow wavelength spectral band, each band being centered on a different wavelength, are carried over a single optical guide. Here xe2x80x9coptical guidexe2x80x9d is used to describe any suitable optical medium, including optical fibres and optical waveguides.
WDM is usually used in combination with established time division multiplexing (TDM) techniques. Hence each WDM channel will typically be used for the transport of a plurality of different messages multiplexed together in the time domain. An add-drop multiplexer (ADM) provides a low cost means for accessing all or a part of the TDM traffic forming a data stream passing along a communications link, such as an optical guide. The traffic passing through the ADM does so via xe2x80x9cline portsxe2x80x9d. Data or messages passing along the communications link are selectively time demultiplexed by switching circuitry in the ADM and passed via so-called tributary ports to their destination. Similarly, data or messages for adding to the communications link are fed to the ADM via the tributary ports and are time multiplexed into the message stream by the ADM switching circuitry. This switching and multiplexing function is performed in the electrical domain. In order to interface to an optical bearer, the ADM includes photo-detectors and laser signal generators.
Currently optical communications networks are made up of rings or meshes comprising electrical repeater/ regenerator/ add/drop electronics linked together by means of optical communications links. These links typically comprise optical fibre. In order to interface to the optical link, the nodes include a laser-based transmitter (electrical to optical converter) and a broad optical-band receiver (optical to electrical converter). WDM is currently becoming established for long haul line systems but has not penetrated the feeder network A xe2x80x98feeder networkxe2x80x99 connects and gathers and consolidates traffic from users and delivers it to the MANs, WANs and LANs e.g. the Access network could be a xe2x80x98feeder networkxe2x80x99. The feeder network is made up of electronic nodes incorporating some switching capability, normally in the form of an electronic ADM. Typically these nodes are designed to handle digital traffic according to the SDH or SONET telecommunications standard protocols but also increasingly ATM and IP protocols are becoming established. At those regions of the network that experience particularly high traffic, the node and path capacities are increased by upgrading these regions to handle higher rate TDM signals e.g. 155 Mbit/s SDH and SONET standards (STM-1) are used widely and 622 Mbit/s (STM-4), 2.4 Gbit/s (STM-16) and 10 Gbit/s (STM-64), are used where necessary.
With the current steep rise in demand for communications there is a need to provide yet greater capacity. The installed optical fibre base will require to be further extended or exploited. The high cost of laying fibre means there is a drive to increase the signalling TDM rate as far as possible and to utilize fully the installed fibres.
As it becomes necessary to run traffic over multiple WDM channels in parallel, new switching needs arise, i.e. when traffic running through one channel is required to couple through another. In addition, the increased switching requirement per node will necessitate additional ADMs per node, with typically one ADM being provided per WDM channel.
With the introduction of optical multiplexers and demultiplexers within these linked networks of ADMs, a single fibre may be used to handle multiple WDM channels so that the net capacity of the segments is enhancedxe2x80x94means to achieve this are described in copending U.S. patent application Ser. No. 09/381,763 in the name of Marconi Communications, filed simultaneously herewith and entitled xe2x80x9cCommunications Systems,xe2x80x9d the entire contents of said application being incorporated herein by reference.
The upcoming situation where multiple rings may be collocated and may need to be connected to other multiple rings brings new requirements for the interconnection of ADMs at nodes. This development will drive the integration level, cost and incorporated switch size of ADMs and thereby the functionality and complexity of complete networks.
There is therefore a need for a more flexible way of transferring traffic between different channels of a wavelength multiplexed optical communications systems.
The present invention provides an optical communications system for communication of a plurality of wavelength multiplexed channels; in which the communications system comprises a node; in which the node comprises a plurality of add drop multiplex (ADM) means, one per channel; in which each ADM means comprises tributary means; in which the ADMs are interconnected by the tributary means.
The present invention also provides a method for switching individual time division multiplexed messages between wavelength multiplexed channels of an optical communications system, comprising the steps of arranging switch means comprising a plurality of add drop multiplex (ADM) means, one per wavelength multiplex channel; in which each ADM means comprises tributary means, the method comprising the steps of interconnecting the ADMs means via the tributary means.
The present invention also provides a method for switching individual time division multiplexed messages between a plurality of streams of time division multiplexed data; in which each stream of time division multiplexed data is comprised in a channel of a wavelength multiplexed optical communications system, the method comprising the steps of providing a node; providing the node with a plurality of add drop multiplex (ADM) means, one per wavelength multiplex channel; providing each ADM means with tributary means and interconnecting the ADM means via the tributary means.
The present invention also provides a means for switching individual messages between a plurality of streams of time division multiplexed (TDM) data in a wavelength multiplexed optical communications system, the system for communication of a plurality of wavelength multiplexed channels; in which each channel comprises one of the streams of TDM data; in which the means comprises switch means; in which the switch means comprises a plurality of add drop multiplex (ADM) means, one per channel; in which each ADM means comprises tributary means; in which the ADMs are interconnected by the tributary means.